1. Field of the Invention
The present invention relates to the identification of a cable buried underground.
2. Summary of the Prior Art
The proliferation of networks of buried cables for many different utilities (electricity, telecommunications, etc) has made it increasingly difficult to identify whether any particular cable, located by a suitable location technique, belongs to a particular utility or not.
Existing identification arrangements fall into two types. Firstly, optical identification techniques involve excavation of the ground in the vicinity of a buried cable, until the cable is visible, and thus can be identified. In order for such an excavation to be carried out, the cable must first be located (i.e. its position determined to enable excavation to take place at the right location), and the excavation thus needed is time-consuming. Furthermore, excavation involves a risk of damage or interference to the buried cable, or possibly to other cables in the immediate vicinity. Moreover, visual identification of cables is not certain. Cables belonging to different utilities can be physically identical, so that visual location systems involve the need for some pre-knowledge of the cables in the vicinity of the excavation site.
It is also possible to identify buried cables by applying an audio frequency electrical signal to the cable, and then detecting the magnetic fields generated by that signal at the surface. This removes the need for excavation, since the frequencies of the signal are chosen so that the magnetic fields generated will be detectable at surface level.
However, such an audio frequency signal may be transferred to other cables by induction and capacitive leakage. This leads to distorted magnetic fields, resulting in mis-location of the cable, or even the possibility of the wrong cable being identified, because induction results in the signal being carried by a cable other than that to which it is applied.
Many different proposals have been made for improving such electromagnetic location, but none have wholly prevented the problem of mis-identification.
U.S. Pat. No. 5,194,812 discloses a method of identifying a cable buried underground in which an alternating current signal is applied to the cable and the magnetic field due to the current signal is detected using a magnetic field sensor brought into proximity with the cable. In U.S. Pat. No. 5,194,812 the alternating signal has a frequency of 10 Hz to 500 KHz.